The present invention relates to an optical communication system for transmitting digital data such as audio data, image data, and text data through an optical transmission path.
FIG. 6 is an entire block diagram showing an example of a conventional optical communication system. Referring to FIG. 6, the optical communication system is constituted by optical terminal station devices 1 and 2 each having the same arrangement and a beam-type repeater 3 connected to the optical terminal station devices 1 and 2 through optical fibers OF1 to OF4, and two optical communication paths for transmitting a main signal (signal transmitted through optical terminal station devices) between the optical terminal station devices 1 and 2 in both the directions.
In an optical transmitter 4 of the optical terminal station device 1, a transmission electric signal serving as a main signal is converted into an optical signal by using an electric/optical converter (E/O) 5, and the optical signal is amplified to a predetermined level by an optical post-amplifier 6. Thereafter, the optical transmitter 4 transmits the main signal to the transmission path (optical fiber OF1). In the repeater 3, the main signal attenuated by loss of the transmission path is amplified by an optical inline amplifier 7 to a predetermined level, and the main signal is transmitted to the transmission path (optical fiber OF2) again. In an optical receiver 8 of the optical terminal station device 2, the main signal attenuated by loss of the transmission path is amplified by an optical pre-amplifier 9, and the main signal is converted into a reception electric signal by an optical/electric converter (O/E) 10 to output the reception electric signal.
As described above, the optical inline amplifier 7 of the repeater 3 does not convert the main signal into an electric signal. For this reason, an arrangement in which an electric signal according to the main signal is detected from the optical inline amplifier 7 and an arrangement in which the optical inline amplifier 7 is supervised and controlled on the basis of the detected electric signal cannot be employed. Therefore, in order to supervise and control the optical inline amplifier 7, an optical signal for supervisory and control (optical supervisory transmission signal (OSC: optical supervised channel)) is used.
In the example shown in FIG. 6, the optical terminal station devices 1 and 2 and the repeater 3 have supervisory transmission signal processing units 11 and 12. The supervisory transmission signal processing unit 11 transmits an OSC having an optical wavelength different from that of the main signal. The OSC includes supervisory/control information of the optical inline amplifier 7. The OSC is optically wavelength-multiplexed with the main signal by an optical system 13, transmitted to the repeater 3, and input to the supervisory transmission signal processing unit 12 through an optical system 14. The supervisory transmission signal processing unit 12 supervises and controls the operation of the optical inline amplifier 7 on the basis of the supervisory/control information included in the OSC. On the other hand, the supervisory transmission signal processing unit 12 outputs an OSC including information of the state, operation, and the like of the optical inline amplifier 7. This OSC is input to the supervisory transmission signal processing unit 11 of the optical terminal station device 1 through optical systems 15a and 16. The supervisory transmission signal processing unit 11 supervises and controls the optical inline amplifier 7 on the basis of information related to the optical inline amplifier 7 included in the OSC. As another method of transmitting the OSC to the repeater 3, a method of superposing the OSC on the main signal to transmit the resultant signal to the repeater 3 in an optical region may be taken.
FIG. 7 is a block diagram of an example of arrangement of the optical inline amplifier 7 shown in FIG. 6. Referring to FIG. 7, the optical inline amplifier 7 is constituted by an optical pre-amplifier (AGC unit) 19, a distribution compensation fiber (DCF) 20, and an optical post-amplifier (ALC unit) 21 which are connected in series with each other. The optical pre-amplifier 19 low-noise-amplifies the main signal by gain constant control (AGC: Auto Gain Control). The DCF 20 compensates for only waveform distortion caused by light distribution in the transmission path. The optical post-amplifier 21 amplifies the main signal amplified by the optical pre-amplifier 19 to a predetermined level by output level constant control (ALC: Auto Level Control).
FIG. 8 is a block diagram of an example of arrangement of the optical post-amplifier 21 shown in FIG. 7. The optical post-amplifier 21 makes an average level of the main signal output from the optical post-amplifier 21 constant by the ALC. The optical post-amplifier 21 is constituted by an optical level controller (optical amplifier) 22, an optical system 23, an O/E 24, a low-pass filter (LPF) 25, and a comparator 26.
The optical level controller 22 amplifies a main signal output from the DCF 20. The optical system 23 partially branches the main signal output from the optical level controller 22 to input the signal to the O/E 24. The O/E 24 optical/electric-converts the optical output signal from the optical system 23. The LPF 25 detects an average level signal of the main signal output from the O/E 24. The comparator 26 compares an average level signal output from the LPF 25 with a reference signal to output the error signal. On the basis of the error signal, the gain of the optical level controller 22 is controlled. The ALC is executed by the above control loop.
The response speed of the ALC is determined by a portion having the lowest response speed in the control loop. This response speed is generally determined by the LPF from the viewpoint of circuit stabilization. In general, a fixed value depending on the level of the main signal output from the optical level controller 22 is set as the reference signal.
To be compared with the ALC, an example of arrangement of the optical pre-amplifier 19 is shown in FIG. 9. As shown in FIG. 9, the optical pre-amplifier 19 controls the gain of an optical level controller 27 by the AGC to be constant. For this reason, the input average level of the main signal is monitored by an optical system 28, an O/E 29, and an LPF 30. The output average level of the main signal is monitored by an optical system 31, an O/E 32, and the LPF 33. The results obtained by the monitoring operations are compared with each other by the comparator 34. The gain of the optical level controller 27 is controlled such that the difference between the levels is constant.
The repeater 3 is arranged to amplify the main signal attenuated by loss of the transmission path to a predetermined level such that the main signal has a waveform being so close to an original waveform as possible. This means that the amplitude of the original main signal is set at the predetermined level, and does not mean that the average level of the main signal is kept constant. Therefore, the ALC is desirably controlled on the basis of the peak detection result of the main signal.
In order to detect the peak of the main signal, a circuit having a speed sufficiently higher than that of the main signal is required. However, the high-speed circuit hinders the superiority of the optical amplifier which has a simple circuit arrangement obtained by amplifying an optical signal and is free from a bit rate. In addition, in a system for optical wavelength division multiplexing (OWDM) transmission in which a plurality of optical signals are multiplexed to amplify the optical signals, peak detection and ALC must be performed in each channel. For this reason, the great advantage of the OWDM in which a plurality of optical signals can be amplified by the optical amplifier at once is lost. For these reasons, average level constant control is generally performed.
The ALC performed by the optical post-amplifier 21 shown in FIG. 8 has the following problem. That is, a main signal transmitted to the repeater 3 is a digital signal transmitted such that logical data of 1/0 corresponds to an ON/OFF state of an optical signal. This main signal is generally subjected to scramble using a pseudo random pattern (PN) to suppress a variation in average level caused by a series of equal signs as hard as possible.
FIG. 10(A) is a graph showing an example of an optical main signal modulated in five stages of a PN (31-bit period). Referring to FIG. 10(A), the ordinate of the graph is standardized with respect to the level of 1/0 of the main signal. FIG. 10(B) is a graph showing an average level corresponding to the main signal shown in FIG. 10(A). A broken line in FIG. 10(B) indicates an average level in a total of 31 bits. A constant value is obtained at 16/31≈1/2. In contrast to this, a solid line in FIG. 10(B) indicates a variation in average level in a total of five bits including two forward bits and two backward bits. In this manner, although the average level of the main signal is constant within a long period of time, the average level varies within a short period of time. The average level of an optical signal actually detected draws a curve being more moderate than that in FIG. 10(B). The optical signal having five stages of a PN has been described above. However, the average level actually varies in a period longer than that of the optical signal.
Here, observation of the average level for a long period of time corresponds to a low response speed of ALC, and observation of the average level for a short period of time corresponds to a high response speed of ALC. For this reason, when the response speed of ALC is too high, as indicated by a solid line in FIG. 10(B), the peak value of the main signal changes every bit. More specifically, signal degradation depending on the variation pattern of the main signal occurs. As a result, the variable part of the main signal on the reception side is removed as noise, and the reception electric signal is different from an original transmission electric signal, so that an error may be generated.
From a viewpoint for avoiding the above problem, the response speed of ALC is desirably set to be sufficiently low. However, when the response speed of ALC is made sufficiently low, a variation in input level of the main signal may not be suppressed. More specifically, even if the response speed of ALC is sufficiently low, a variation in input level of a main signal having a very low speed (e.g., temperature drift of loss of the transmission path) can be suppressed. In contrast to this, a variation in input level of a main signal having a relatively high speed (e.g., bending loss generated by vibration of a transmission path (optical fiber) may not be suppressed. As described above, the response speed of ALC is basically traded off the variation in input level of the main signal.
It is an object of the present invention to provide an optical communication system and an optical signal control method therefor capable of suppressing degradation of an optical signal caused by the response speed of ALC in a repeater compared with a prior art.
The present invention employs the following arrangement to achieve the above object. According to the present invention, there is an optical communication system comprising an optical transmission device for converting an electric-signal-type main signal into an optical-signal-type main signal to transmit the optical-signal-type signal, and a repeater for receiving a main signal from the optical transmission device to transfer the main signal to a reception device for a main signal. The repeater has an automatic level controller for performing average level constant control to the main signal received from the optical transmission device and outputting the main signal toward the reception device. The optical transmission device has an average level detector for detecting an average level detection signal, representing an average level of a main signal obtained immediately after the electric signal type is converted into the optical signal type, from the main signal. The average level detector has a detection band width being equivalent to the response speed of the automatic level controller. The automatic level controller performs average level constant control with reference to the average level detection signal detected by the average level detector.
According to the present invention, the automatic level controller of the repeater performs average level constant control to the main signal with reference to the average level detection signal. In this manner, a sharp change in input level of the main signal received by the repeater is suppressed by the automatic level controller. When the response speed of the automatic level controller is set at such a level that signal degradation depending on the variation pattern of the main signal does not occur, in the repeater, the main signal can be suppressed from being degraded by the cause of the response speed of the automatic level controller. Therefore, an optical signal being more appropriate than that of the prior art is transmitted toward the reception device, and an error caused by signal degradation is suppressed from being generated.